Field of Invention
The present invention relates generally to methods and systems for integrating a mode converter, waveguides, and an on-chip device such as a detector on a single substrate.
Background
Optical waveguides and waveguide devices have enabled optical telecommunications, by providing ways of guiding light through a medium and by performing useful operations such as distinguishing different wavelengths from a single optical signal. These traditional waveguide structures, having a low index difference between core and cladding layer, are typically large in its mode field, and they impose a fabrication constraint for integrating with current state-of-the-art semiconductor circuits due to relative size differences. Recent advances in the fabrication process and design of waveguides with high index difference between core and cladding layer have enabled waveguides and waveguide devices to be miniaturized in planar lightwave circuits (xe2x80x9cPLCxe2x80x9d).
The on-chip waveguides used to miniaturize PLCs can be high index difference waveguides. These high index difference waveguides have smaller bending radii than large mode field size waveguides and low index difference waveguides, and therefore bending and splitting of propagating light can be implemented in smaller areas using such high index difference waveguides.
The miniaturization of PLCs on a similar scale as semiconductor integrated circuits enables integration of optical waveguides, optoelectronics, and electronic integrated circuits in one chip. However, as the size of on-chip waveguides and waveguide devices is miniaturized, the optical mode in the on-chip waveguides is mismatched with the optical mode in external fibers. Typically, external fibers, having a waveguide core of approximately 10 micron in diameter, have large mode field sizes, therefore low index difference waveguides, whereas a high index difference waveguide has substantially smaller mode field. This mismatch in mode size between an external fiber and an on-chip waveguide creates a connection loss between the PLC chip and the external fiber when the two are directly connected. An abrupt change in the refractive index at the interface between an external fiber, a low index waveguide, and an on-chip high index waveguide, also causes transmitted power loss due to the reflection of the light wave signal.
To resolve the connection loss between a miniaturized waveguide and an external fiber, an on-chip mode converter can be used to reduce this connection loss, as shown in U.S. patent application Publication No. 20020031296 A1. Such an on-chip mode converter has a thick lower cladding (cladding between a substrate and the waveguide core) and a thick upper cladding (cladding above the core). The thick cladding layers are needed for the low index difference part of the waveguide. Because of the thick lower cladding, the waveguide core is many microns away from the substrate.
Such a light wave mode conversion concept is also shown in publications such as in IEEE Photonics Technology Letters, Vol. 5, No. 9, September 1993 by Brenner et al., in IEEE Photonics Technology Letters, Vol. 7, No. 5, May 1995 by Zengerle et al., in Electronics Letters, Vol. 29, No. 4, February 1993, by Schwander et al., in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 3, No. 6, December 1997 by Moerman et al., in Proceedings of SPIE, Vol. 4870, 2002 by Dutta et al., and in U.S. Pat. No. 5,199,092 issued to Stegmueller et al. Many of these optical mode converting structures require thick lower cladding and upper cladding layers as the one described earlier, causing the waveguide core to be many microns away from the substrate. Due to such a geometric constraint, there is a difficulty in integrating the waveguide with an on-chip device. On-chip devices are typically built relatively close to a substrate, and since the waveguide core is many microns away from the substrate, there exists a rather large distance between the waveguide core and an on-chip device. Coupling light from the waveguide core through the large distance to an on-chip device is difficult. Even for PLCs with just low index contrast waveguides, the distance between the waveguide core and the on-chip device is large. Therefore similar types of geometric constraints exist for integrating low index contrast waveguides with on-chip devices.
The present invention is directed to an integrated device that includes a waveguide and an on-chip device formed on an optical chip so that an efficient coupling is made between the waveguide and an on-chip device by forming a region surrounded by metal, where the waveguide terminates in. In one embodiment, the region is formed at the end of a waveguide that encloses the end of the waveguide and an on-chip detector. A light coming from the core of a waveguide is essentially trapped inside a region surrounded by metal and directed to the on-chip detector for coupling of light.
The present invention is also directed to an integrated device that includes a low index difference waveguide, an on-chip mode converter, a high index difference waveguide, and an on-chip function formed on an optical chip, so that these are built on a single substrate at different surface heights, making the high index difference waveguide close to the substrate surface. In one embodiment, a trench is formed on one end of the substrate, and the mode converter and low index difference waveguide can be formed on the surface of this trench. The high index difference waveguide can be formed either on the unprocessed surface of the substrate or on the surface of a trench that is relatively shallower than the one used for the mode converter and low index difference waveguide. The on-chip function can be formed on the unprocessed surface of the substrate. This allows the core of the high index difference waveguide to be close to the surface of the substrate. By having the high index difference waveguide in close proximity to the surface of the substrate, devices such as on chip Ge detectors and electronics can be integrated on a chip with the waveguide that is connected to the mode converter.
Different on-chip functions can be integrated using the aspects of the invention described above and in more detail below. An example would be an on-chip Ge detector, and using the aspects of the invention described, the integration of a mode converter, high index difference waveguide and a Ge detector on a silicon substrate is possible.
This invention accordingly comprises the features of construction, combination of elements, arrangement of parts, which will be exemplified in the disclosure.